Transmissions are known from practice that have friction-locking shift elements and/or form-locking shift elements as shift elements. The present invention relates to a method for actuating a form-locking shift element of a transmission, wherein the transmission either exclusively uses form-locking shift elements, or form-locking shift elements and friction-locking shift elements in combination.
In order to properly engage a previously disengaged form-locking shift element when executing a gearshift from a current gear to a target gear, the form-locking shift element must be engaged or meshed within a specified rotational speed range or rotational speed window around a specified differential rotational speed. Otherwise, there exists the danger that the form-locking shift element cannot be engaged, or becomes damaged upon engaging.
To ensure the engagement of a form-locking shift element within the specified rotational speed window, the procedure known in practice for transmissions with at least one form-locking shift element is to determine an actuation time for a form-locking shift element to be engaged while shifting gears depending on the reaction time of the assemblies participating in the control of the form-locking shift element, depending on an engaging time of the form-locking shift element, and depending on at least one filtered signal, so that the form-locking shift element engages within the specified rotational speed window. The engaging time required to engage the form-locking shift element is, for example, dependent on a transmission fluid temperature, and when the transmission fluid temperatures are low, the engaging time is relatively long, and when the transmission fluid temperatures are high, the engaging time is relatively short.
The problem with such an actuation of a form-locking shift element of a transmission to be engaged is that changes can occur in the drivetrain directly before an actuation time determined in this manner, as well as directly after this actuation time, that make it impossible to engage the form-locking shift elements within the specified rotational speed window. A sudden increase or decrease in load from a driver actuating the gas pedal can, for example, suddenly change the rotational speed at the transmission input side. A sudden actuation of the brake pedal can also suddenly change the rotational speed at the transmission output side. Furthermore, a rotational speed at a transmission output side can, for example, suddenly change when an anti-lock braking system or anti-skid control fails.
Since filtered signals are used when determining the actuation time for a form-locking shift element to be engaged when shifting gears, such sudden signal changes are not sufficiently taken into account, when determining the actuation time, to subsequently ensure a smooth engagement of the form-locking shift element. If these signal changes only occur after the actuation time for the form-locking shift element, there is also no opportunity, according to the prior art, to influence the actuation of the form-locking shift element.
A method is known from DE 10 2009 056 793 A1 for executing a shift in an automatic transmission of a motor vehicle in which a differential rotational speed can be forced into the range of the specified rotational speed window for a form-locking shift element such that the rotational speed difference of the form-locking shift element is reduced by engaging at least one friction-locking shift element of the transmission.